In typical prior art optical systems, such as telescopes or microscopes, the means for focusing a beam of light can be either reflective or refractive. In a refractive system, a glass lens is typically used to focus light at a focal point. While a glass lens will adequately transmit visible light along with a small region of the infrared radiation spectrum, a glass lens cannot adequately focus a wide range of the radiation spectrum including the lower infrared frequencies and radar frequencies. Additionally, glass lenses are opaque to radiation at these lower frequencies. Moreover, it is not always possible to use a glass lens in environments which are hostile to glass. Examples of such environments include high nuclear radiation environments, wherein the glass lens can suffer damage due to irradiation and eventually become milky and opaque. Other harsh environments for glass lenses include extreme thermal gradients, and in situations where mechanical shock is present.
In a typical reflective type system, such as a reflecting telescope, a pair of mirrors may be arranged such that those light rays which pass around the outer periphery of the first mirror, are reflected off of a second mirror (which is concave), and directed back toward the first mirror, which is convex. The light rays then are reflected by the first mirror and brought to a focus at a focal point. If the focal point is beyond the second mirror, the light rays can pass through an opening constructed in the second miror to convege at that focal point. Alternatively, the light rays may first reflect off a curved convex mirror, get directed back toward the concave second mirror, to be brought to a focus at a focal point after passing around the outer periphery of the first mirror. FOr these arrangements the light rays are obstructed in the middle of the beam of light by at least one of the mirrors. For a telescope, this region of obstructed light rays is not important because of the great sizes and distances of the observed objects. Moreover, in the second type of arrangement rays near the center line may pass around the first lens and reach a focal point, but must do so at a final angular deviation.
It would therefore be advantageous to provide a focusing device which will operate for any type of radiation beam. Such a device would be able to overcome the limitations of a glass lens, such as the chromatic aberration associated with the index of refraction of the glass in a glass lens. In operating in the lower frequencies, such as with infrared radiation, it is desirable to limit the absorption of any light to a minimum. Certain devices, such as night vision devices, operate in the infrared range. Therefore, absorption of light in such a device is to be minimal, if at all. Additionally, such a device which will operate in a harsh environment without affecting the focal qualities of the lens would be desirable.